The term “transmitter” is used herein in a broad sense to denote any device capable of transmitting data over a serial link or other link, and optionally also capable of performing additional functions which can include encoding and/or encrypting the data to be transmitted. The term “receiver” is used herein in a broad sense to denote any device capable of receiving data that has been transmitted over a serial link or other link, and optionally also capable of performing additional functions, which can include decoding and/or decryption of the received data, and other operations related to decoding, reception, or decryption of the received data. For example, the term receiver can denote a transceiver that performs the functions of a transmitter as well as the functions of a receiver.
The expression “serial link” is used herein to denote a serial link (having any number of channels) or a channel of a serial link, where the term “channel” of a serial link denotes a portion of the link that is employed to transmit data in serial fashion (e.g., a conductor or conductor pair between a transmitter and receiver over which data are transmitted serially, either differentially or in single-ended fashion).
There are various, well-known serial links for transmitting video data and other data. One conventional serial link is known as a transition minimized differential signaling interface (“TMDS” link). This link is used primarily for high-speed transmission of video data from a set-top box to a television, and also for high-speed transmission of video data from a host processor (e.g., a personal computer) to a monitor. Among the characteristics of a TMDS link are the following:
1. video data are encoded and then transmitted as encoded words (each 8-bit word of digital video data is converted to an encoded 10-bit word before transmission);
2. the encoded video data and a video clock signal are transmitted as differential signals (the video clock and encoded video data are transmitted as differential signals over conductor pairs without the presence of a ground line); and
3. three conductor pairs are employed to transmit the encoded video, and a fourth conductor pair is employed to transmit the video clock signal.
Another serial link is the “High Definition Multimedia Interface” interface (“HDMI” link) developed Silicon Image, Inc., Matsushita Electric, Royal Philips Electronics, Sony Corporation, Thomson Multimedia, Toshiba Corporation, and Hitachi. It has been proposed to transmit encrypted video and audio data over an HDMI link.
Another serial link is the “Digital Video Interface” (“DVI” link) adopted by the Digital Display Working Group. It has been proposed to use the cryptographic protocol known as the “High-bandwidth Digital Content Protection” (“HDCP”) protocol to encrypt digital video data to be transmitted over a DVI link, and to decrypt the encrypted video data at the DVI receiver. A DVI link can be implemented to include two TMDS links (which share a common conductor pair for transmitting a video clock signal) or one TMDS link, as well as additional control lines between the transmitter and receiver. We shall describe a DVI link (that includes one TMDS link) with reference to FIG. 1. The DVI link of FIG. 1 includes transmitter 1, receiver 3, and the following conductors between the transmitter and receiver: four conductor pairs (Channel 0, Channel 1, and Channel 2 for video data, and Channel C for a video clock signal), Display Data Channel (“DDC”) lines for bidirectional communication between the transmitter and a monitor associated with the receiver in accordance with the conventional Display Data Channel standard (the Video Electronics Standard Association's “Display Data Channel Standard,” Version 2, Rev. 0, dated Apr. 9, 1996), a Hot Plug Detect (HPD) line (on which the monitor transmits a signal that enables a processor associated with the transmitter to identify the monitor's presence), Analog lines (for transmitting analog video to the receiver), and Power lines (for providing DC power to the receiver and a monitor associated with the receiver). The Display Data Channel standard specifies a protocol for bidirectional communication between a transmitter and a monitor associated with a receiver, including transmission by the monitor of Extended Display Identification (“EDID”) data that specifies various characteristics of the monitor, and transmission by the transmitter of control signals for the monitor. Transmitter 1 includes three identical encoder/serializer units (units 2, 4, and 5) and additional circuitry (not shown). Receiver 3 includes three identical recovery/decoder units (units 8, 10, and 12) and inter-channel alignment circuitry 14 connected as shown, and additional circuitry (not shown).
As shown in FIG. 1, circuit 2 encodes the data to be transmitted over Channel 0, and serializes the encoded bits. Similarly, circuit 4 encodes the data to be transmitted over Channel 1 (and serializes the encoded bits), and circuit 6 encodes the data to be. transmitted over Channel 2 (and serializes the encoded bits). Each of circuits 2, 4, and 6 responds to a control signal (an active high binary control signal referred to as a “data enable” or “DE” signal) by selectively encoding either digital video words (in response to DE having a high value) or a control or synchronization signal pair (in response to DE having a low value). Each of encoders 2, 4, and 6 receives a different pair of control or synchronization signals: encoder 2 receives horizontal and vertical synchronization signals (HSYNC and VSYNC); encoder 4 receives control bits CTL0 and CTL1; and encoder 6 receives control bits CTL2 and CTL3. Thus, each of encoders 2, 4, and 6 generates in-band words indicative of video data (in response to DE having a high value), encoder 2 generates out-of-band words indicative of the values of HSYNC and VSYNC (in response to DE having a low value), encoder 4 generates out-of-band words indicative of the values of CTL0 and CTL1 (in response to DE having a low value), and encoder 6 generates out-of-band words indicative of the values of CTL2 and CTL3 (in response to DE having a low value). In response to DE having a low value, each of encoders 4 and 6 generates one of four specific out-of-band words indicative of the values 00, 01, 10, or 11, respectively, of control bits CTL0 and CTL1 (or CTL2 and CTL3).
In operation of the FIG. 1 system, a cable comprising connectors 20 and 21 and conductors (wires) 22 is connected between transmitter 1 and receiver 3. Conductors 22 include a conductor pair for transmitting serialized data over Channel 0 from encoder 2 to decoder 8, a conductor pair for transmitting serialized data over Channel 1 from encoder 4 to decoder 10, a conductor pair-for transmitting serialized data over Channel 2 from encoder 6 to decoder 12, and a conductor pair for transmitting a video clock over Channel C from transmitter 1 to receiver 3. Conductors 22 also include wires for the DDC channel (which can be used for bidirectional I2C communication between transmitter 1 and receiver 3), a Hot Plug Detect (HPD) line, “Analog” lines for analog video transmission from transmitter 1 to receiver 3, and “Power” lines for provision of power from transmitter 1 to a receiver 3.
Other serial links include the set of serial links known as Low Voltage Differential Signaling (“LVDS”) links (e.g., “LDI,” the LVDS Display Interface), each of which satisfies the TIA/EIA-644 standard or the IEEE-1596.3 standard, ethernet links, fiberchannel links, serial ATA links used by disk drives, and others.
During high-speed data transmission over a cable, the cable itself introduces losses and dispersion which reduce the signal quality at the receiver end. High-speed serial communication makes it possible to transfer high-speed data over a single conductor or conductor pair. However, as one or both of the frequency of transmitted signal and the cable length increases, the distortion due to frequency dependent delay and attenuation can make the eye at the receiver almost unusable. Also, handling of the cable itself becomes difficult for typical users in consumer applications.
Frequency dependent attenuation not only attenuates signals but also generates dispersion. These artifacts increase the chance of false detection of received signals. The most important parameter for the receiver is the eye opening at the receiver. A larger eye opening is correlated with better signal quality. Major sources of signal distortion are frequency dependent attenuation, imperfect impedance matching, far end cross talk and EMI. For relatively low frequency signals, various signal processing techniques (e.g., adaptive equalization) have been used to compensate for signal distortion. However, for higher frequency signals, especially those indicative of an NRZ (non return to zero) data stream, equalization becomes more difficult if the cable characteristics are not well defined or known. By using sophisticated methods, cable characteristics can be deduced using circuitry in a transmitter and/or receiver (with the cable connected between the transmitter and receiver) but this requires complex handshaking and signal processing circuitry.
Transmission of signals indicative of data (e.g., signals indicative of video or audio data) to a receiver over a cable degrades the data, for example by introducing time delay error (sometimes referred to as jitter) to the data. In effect, the cable applies a filter (sometimes referred to as a “cable filter”) to the signals during propagation over the cable. The cable filter can cause inter-symbol interference (ISI).
Equalization is the application of an inverted version of a cable filter to signals received after propagation over a link. The function of an equalization filter (sometimes referred to as an “equalizer”) is to compensate for, and preferably cancel, the cable filter. A transmitter can implement “pre-emphasis” equalization by applying relatively greater amplification to some data values of a sequence of data values to be transmitted, and relatively less amplification to other data values of the sequence. A receiver can also implement an equalization filter.
In a system for transmitting data over a cable from a transmitter to a receiver, either or both of the transmitter and receiver can perform equalization. In many such systems, the user can couple any of a variety of cables between the transmitter and receiver and can swap one cable for another (e.g., one of different length) when desired. A set of equalization parameters suitable for use with one cable would often be unsuitable for use when this cable is replaced by another cable (e.g., a much shorter or much longer cable). Until the present invention it had been time-consuming and/or expensive to determine an optimal (or suitable) set of equalization parameters for an equalization filter in a transmitter or receiver of such a system (for example, since characterization of cable properties had required complex handshaking and signal processing circuitry, as noted above).